In order to achieve optimum, linear polarised patterns from an axially symmetric reflector or for an offset reflector, ideal field intensities and polarization at a focal plane of the reflector are generated. Ideal focal plane field intensities and polarisation are commonly generated using a scalar feed corrugated horn. One such horn is shown in U.S. Pat. No. 4,349,827, entitled "Parabolic antenna with horn feed array" to Bixler et al. Horns are designed to provide ideal focal plane field intensities and polarisation. Unfortunately, horns are fixed, three-dimensional structures thereby increasing antenna fragility and size.
Further, the use of a scalar feed horn for transmission requires a solid state power amplifier (SSPA) incorporating a power combiner at an output of parallel amplifiers. The combiner enables power delivery to the horn. A typical loss in the output combiner is 1.5 dB for a 14 GHz signal--or, 6 W for a 20 W feed.
It was proposed by K. Woo, in an article entitled "Array-fed reflector antenna design and applications", Second International Conference on Antennas and Propagation. 13-16 Apr., 1981, Part 1: Antennas, pp. 209-213, and by A. W. Rudge, in an article entitled "Current trends in antenna technology and prospects for the next decade", IEEE Antennas and Propagation Society Newsletter, Vol. 25, No. 6, December 1983, pp. 5-12, to use an active feed comprising parallel amplifiers feeding a rectangular array of equally spaced radiating elements to generate a signal having a same focal plane field pattern as a scalar feed; the loss is typically 0.5 dB. The lower amount of loss allows a similar number of amplifiers to provide more power than a same number of amplifiers driving a scalar feed horn, or allows elimination of one or more parallel amplifiers resulting in a same output power. The assembly of an active feed having an array of equally spaced radiating elements is less costly than that of a solid state power amplifier.
The benefits of the proposed designs are significant but, unfortunately, it is near impossible to match ideal focal plane field intensities and polarisation using the proposed active feed. Invariably, side lobes and other aberrations in the feed signal occur. Further, a number of amplifiers having different amplifications are necessary for such an active feed.